Changing the propagation path of a beam of optical radiation is a basic task for a large variety of optical applications. As an example we may consider the task in which light from a point-like source must be collimated, reflected to an angle, and focused into an image of said point-like source. This task is frequently encountered in such measurement devices where incident light from a light source is directed to a sample, and the optical emissions generated in the sample are measured and analyzed. In some applications a corresponding task only occurs in handling one of these directions, i.e. either the incident light or the optical emissions.
FIG. 1 illustrates schematically a known case in which a light source arrangement 101 comprises an essentially point-like light source (not separately shown). The emitted light is spatially limited so that what comes out of the light source arrangement 101 is an essentially conical beam of light. A first convex (or planoconvex, like in FIG. 1) lens 102 is used as a collimator that converts the diverging, conical beam of light into a cylindrical beam of collimated light. A mirror 103 reflects the cylindrical beam into a different direction. The angle 104 between the propagation direction of the incident beam and the propagation direction of the reflected beam is called the reflection angle. It is here 90 degrees but could as well be something else. The reflected beam passes through a second convex lens 105, which focuses it onto a target in an image arrangement 106, which is so called because if the focusing is appropriate, an image of the light source can be obtained in the image arrangement 106.
A drawback of the prior art arrangement of FIG. 1 is the relatively large number of optical material/surrounding material interfaces that the light must encounter on its path. Even in very high quality lenses some optical losses take place due to boundary reflections between the lens and the substance surrounding it. In lenses and mirrors also other optical effects take place, such as scattering, which all reduce the amount of transmitted light. Another drawback is related to the relative sensitivity to rough handling of a structure that has multiple separate optical elements that need to be accurately located and aligned.
FIG. 2 illustrates another prior art arrangement, in which the number of glass/air interfaces has been reduced from that of FIG. 1. The difference to FIG. 1 is the use of only one convex lens 202 between the light source arrangement 101 and the mirror 103. The focal length of lens 202 is selected so that the image of the light source is focused to the image arrangement 106. Drawbacks of this solution may include the increased distance needed between the light source arrangement 101 and the lens 202. Also the reflection coefficient from the mirror surface is significantly smaller than 1, whereas the total internal reflection corresponds to the reflection coefficient 1.